1. Field of the Invention
The present invention pertains to an inside diameter blade for cutting ingots of material such as silicon or gallium arsenide.
2. Prior Art
FIGS. 1 and 2 of the accompanying drawings depict a conventional inside diameter blade which comprises an annular stainless steel plate 1 having a cutting edge 2 comprised of a layer of abrasive grains deposited along the internal circumference of the annular stainless steel plate 1. The layer of abrasive grains is comprised of ultrafine abrasive grains of diamond, cubic boron nitride (cBN), or the like dispersed in a plating phase of nickel (Ni), cobalt (Co), or the like. The inside diameter blade is securely fixed at its outer peripheral portion to a drive apparatus so that it can be rotated at high speed. An ingot of silicon or gallium arsenide is inserted through an opening of the inside diameter blade and is cut by the cutting edge 2 into wafers.
The inside diameter blades hitherto used are provided with annular plates 1 of the following nominal outer diameters D and thicknesses T:
TABLE 1 ______________________________________ Nominal outer diameter of annular Thickness plate D (inch) T (mm) D/T ______________________________________ 16 5/8 0.10 4220 21 1/2 0.12, 0.13 4550, 4200 23 1/2 0.13 4585 27 1/6 0.15 4600 ______________________________________
When cutting an expensive material such as silicon, the cutting margin must be diminished as much as possible in order to minimize waste of the material to be cut. In order to reduce the cutting margin, the maximum thickness of the layer of abrasive grains in the axial direction of the annular plate 1 should be decreased. However, if only the thickness of the layer of abrasive grains is reduced, the inside diameter blade becomes susceptible to an increased cutting load due to friction between the annular plate 1 and the material to be cut. In addition, chips produced during the cutting operation cannot be smoothly removed. Therefore, the thickness T of the annular plate 1 must as well be reduced.
However, if the thickness T of the annular plate 1 is reduced to less than 1/5,000 of its outer diameter D, the rigidity of the annular plate 1 becomes insufficient, and the internal circumference of the annular plate 1 then tends to vibrate during the cutting operation, thereby resulting in a lowering of the cutting accuracy with such effects as a fluctuation in wafer thickness.
As a possible solution to this problem, an inside diameter blade in which the annular plate has a tensile strength of no less than 180 kgf/mm.sup.2 was proposed. With this design, the inside diameter blade is less susceptible to vibration. However, after further investigation, it was realized that another disadvantage arises. Specifically, when the blade of the above construction is used to cut wafers from ingots, the resulting wafers are warped as depicted at W.sub.1 in FIG. 3. The wafer W.sub.1 thus warped is compressed as shown by the arrows in FIG. 3 in a following lapping process and is lapped as designated by the two dot and dash line. The wafer, however, returns to its warped form when the compressing force is removed after the lapping operation. Thus, warpage of wafers cannot be avoided.
Japanese Patent Application A-Publication Nos. 61-114813 and 61-106207 describe methods of getting rid of the warpage of wafers (FIG. 4). In this method, an end face 4a of an ingot 4, from which wafers have been cut, is ground by a face grinding device 5 to obtain a wafer W.sub.2 with one planar face 6 as illustrated in FIG. 5. Then, the other face 7 of the wafer W.sub.2 is lapped with the planar face 6 resting on a reference face and a wafer without warpage as shown at W.sub.3 can thus be obtained.
In the above method, however, the face grinding device 5 must be attached to the slicing machine, and therefore, the slicing machine becomes more intricate in structure. Furthermore, since an additional grinding operation is necessary, the slicing operation is not efficient and the yield of wafers is lowered.